Physics Unit 4 Jan 2025 Past Paper PDF

Summary

This document contains physics past paper questions related to magnetism, electromagnetism, and the special theory of relativity. It includes topics like black holes, the magnetic field, magnetic flux, and magnetic force on a current-carrying wire. The paper likely covers topics relevant to a secondary school physics curriculum.

Full Transcript

- If the muons are produced in the upper atmosphere (15-20 km up) and travel, on average, a distance of about 660 m then not many should be capable of reaching the ground. - The measured intensity/high-count rate (74%) of muons at ground level is too high according to this calculation. - In...

- If the muons are produced in the upper atmosphere (15-20 km up) and travel, on average, a distance of about 660 m then not many should be capable of reaching the ground. - The measured intensity/high-count rate (74%) of muons at ground level is too high according to this calculation. - In a non-relativistic scenario, the time needed to reach the ground is greater. Under this scenario most muons would decay before reaching the ground. - However, using Einstein's special theory of relativity, when a muon moves at near the speed of light the lifetime is increased by a factor of ten or more. t= d/c= 1.5x103/ 3 x 108 = 5.5x10-5 s - Hence, muons are capable of reaching ground level Black Holes: - Black holes have such high gravitational field strength that nothing, including light, can escape from within them. - It has been suggested that black holes can lose mass over time. - Energy from a black hole allows the production of a particle-antiparticle pair outside the black hole. - The two particles move off in opposite directions. One particle falls into the black hole and the other escapes. - Therefore the energy of the escaping particle is lost by the black hole. - The particles created are a muon and an anti-muon, each with mass 106 MeV/c. - This is the minimum mass lost by the black hole - The minimum value assumes no kinetic energy is carried away by the particle A particle with kinetic energy would require more energy from the black hole and hence a greater mass decrease from the black hole Magnetism The magnetic field → A region where a magnetic force is exerted on a moving charge or current carrying conductor (Any charge at rest, it experiences no force) Magnetic flux ɸ (Magnetic Field lines) → The product of the magnetic flux density normal to the surface and the area of the surface. Webber Wb Properties: - Shows the direction of the magnetic force on north magnetic pole at that point. - The relative strength at different points in the field is shown by the separation of the field lines → Closer the lines, the stronger. The further the weaker. The field is stronger closer to magnet and gets weaker as distance increases; at great distances it is negligible (Higher current, closer lines, larger magnetic flux density) - Lines go from the North Pole to the South Pole - Equally spaced and parallel - They come out from the N pole and into the S pole - Never cross/Intersect each other → Since, field is unique at any point - Are continuous - The direction of the magnetic field is tangent to the field line at any point in space. (A small compass will point in the direction of the field line) - The strength of the field is proportional to the number of lines per unit area perpendicular to the lines Neutral Points → Forced due to the 2 magnets are exactly equal and opposite → Representing field lines: (A) Iron filings 1. Place hard thin piece of plastic on magnet 2. Sprinkle iron filings 3. Tap the plastic gently 4. Iron filings will be arranged in pattern similar to magnetic field pattern 5. Use plotting compass to find the direction of field lines (B) Plotting compass (They are best represented on bar magnets, consist of N pole and S pole on the other side) - The magnetic field is produced on a bar magnet by the movement of electrons within the atoms of the magnet - This is a result of the electrons circulating around the atoms, representing a tiny current and hence setting up a magnetic field - The direction of a magnetic field on a bar magnet is always from north to south Magnetic flux density B (Magnetic Field strength) I → The amount of magnetic flux passing through a unit area at right angles to the magnetic field lines Vector Quantity - Wbm-2 → Tesla T 1 Wbm-2 = 1 T Magnetic flux = Magnetic flux density x Area through which flux passes ɸ = B⦜xA Magnetic flux linkage 𝛴 → The magnetic flux linkage through a coil is the product of the number of turns of the coil and the magnetic flux density normal to the surface and the area of the surface. 𝛴 = NBA Flux linkage = Number of turns x Magnetic flux density = NBA No. of turns of a coil (N) = Length of coil / Thickness of each turn 1. 𝛴 = NBAcosⲑ → Where ⲑ is the angle between the normal to the coil and the direction of 2. 𝛴 = NBAsinⲑ magnetic field → Where ⲑ is the angle between the coil and the direction of magnetic field 𝛴 max when coil is perpendicular to field, ⲑ=90° sin90=1 𝛴 zero when coil is parallel to field, ⲑ=0 sin0=0 3. Δ𝛴 = N.Δɸ = NBA(ɸ1-ɸ2) = NBA(sinⲑ1-sinⲑ2) → Change in magnetic flux, when the coil is moved across magnetic field. The area swept is the change in area Magnetic materials Non-magnetic materials Called Ferromagnetic Called Non- ferromagnetic Can be magnetised Cannot be magnetised Examples: Examples: - Iron (Steel & Soft iron) - Aluminium - Nickel - Copper - Cobalt - Gold - Mu-metal (Ni-Fe alloy) - Silver - Lead Soft magnetic materials Hard magnetic materials Can be magnetised and Can be magnetised and demagnetised easily demagnetised hardly Form electromagnets Form permanent magnets Magnetisation methods 1. Electrical method: Place steel bar in solenoid, connect solenoid to battery. Switch on Current for a few seconds then switch it off. Demagnetisation Methods 2. Electrical method: Place steel bar in solenoid, connect solenoid to alternating current. While AC is flowing, slowly withdraw magnet from solenoid in W-E direction (or) reduce current in solenoid slowly The Magnetic effect of a current → Magnetic field is stronger at A than at B → Field lines at B can’t pass though Q, because they repel each other as they are along the same direction To increase the strength of magnetic field around a current carrying wire: - Increase the current To reverse the direction of magnetic field around a current carrying wire: - Reverse the current Exp 3: Show the shape of magnetic field lines and the direction of field lines: 1. Pass straight wire vertical to plane of the paper 2. Let current pass through wire 3. Place the compass needle near the conductor and mark the position of its head and tail 4. Follow the path of the needle 5. Join all the path of the needle 6. Repeat using are positions of needle Two parallel straight wires: (a) Carrying current in the same direction, Attract - Force due to the interaction between the magnetic field of the two wires (resulting in combined magnetic field) - Relatively weaker field in the space between the wires, means that the forces produced are pushing wires closer together - Direction of force can be determined by FLHR (b) Carrying current in the opposite directions, Repel - Force due to the interaction between the magnetic field of the two wires (resulting in combined magnetic field) - Relatively stronger field in the space between the wires, means that the forces produced are pushing pushing wires apart - Direction of force can be determined by FLHR → Fleming left hand rule: (Placed in field, Current → Moved, causes KE) First finger: Direction of field Second finger: Direction of current Thumb: Direction of force (Thrust) Magnetic force on a current carrying wire : (a) A current carrying wire moves when placed in an magnetic field: - Magnetic field due to current interacts with magnetic field of magnet - Producing a force which acts on the wire - Direction of force can be determined using FLHR (b) The force is proportional to: - Current in wire - Length of conductor in field - sinⲑ, the angle conductor makes with field → If the conductor is parallel to the magnetic field, there is no force - Strength of field / Magnetic flux density → If direction of field or current is reversed, the direction if motion/force is also reversed Magnetic force: F=BILsinⲑ B= magnetic flux density L= Length of wire in field ⲑ = Angle between the conductor and field Magnetic flux density B (Magnetic Field strength) II → The magnetic force passing acting on a unit length of a wire carrying current of 1A placed at right angles/perpendicular to the field B=F/IL Vector Quantity - Kgms-2 → Wbm-2 → Tesla T One Tesla is: The magnetic field density which in a wire carrying current of 1A produces a force of 1N for each 1m of wire placed perpendicular to the field Solenoid: (a) Direction of the current round the ends of the coil: → If the coil flows in the first turn in anticlockwise direction: This end is the north pole → If the coil flows in the first turn in clockwise direction: This end is the south pole To increase the strength of magnetic field around a solenoid: - Increase the current - Increase number of turns - Use soft iron core To reverse the direction of magnetic field around a solenoid: - Reverse the current Electromagnets → Piece of soft iron inside a solenoid connected to DC supply To increase the strength of magnetic field of an electromagnet: - Increase the current - Increase number of turns - Use soft iron core Advantages of electromagnets: - Can be easily demagnetised by switching of the current - The strength can be controlled, by changing magnitude of current Uses: - To lift and shift iron scrap in junk yards - In an electric bell - In Cir cute breakers - In Relay stitches - In telephone receivers Exp 4: Investigate how a magnetic field affects a wire carrying an electric current: ⲑ as variable→ 1. Turn the electronic balance on with everything in place without current running through the circuit. (easily done by turning off the power supply) 2. Re-zero the balance to zero. There should be a set/zero button to the left of the screen on the balance. 3. Place a single straight wire, connected to a dc power supply in series with an ammeter and a rheostat, is taped to a metre rule and clamped horizontally at right angles to the field. 4. Connect the circuit by turning on the power supply. 5. In small intervals, we chose 15º, measure the magnetic force downward on the balance with each respective angle. Measure from 0º up to 90°. 6. Record the data to create a graph of mMagnetic field vs sinⲑ Current as variable→ 3. Clamp the wire horizontally at right angles to the field. ⲑ=90° 4. Vary the current using the power source and record the weight on the balance. 5. Plot a graph magnetic force versus current. Length as variable→ 2. Clamp the wire horizontally at right angles to the field. ⲑ=90° 3. Vary the length of the wire using wires of different lengths and record the weight on the balance. 4. Plot a graph magnetic force versus length of wire. If the reading in the wire increases: - There is a force into the page acting on magnet (FLHR not valid here, force on magnet not wire) - The force is acting downwards - By N3 law, there is an equal and opposite force acting upwards on wire - Using FLHR, direction of field and current can be determined DC Motor Electrical Energy → Kinetic/Mechanical energy Structure: 1) Magnet 2) Coil of wire around a soft core iron 3) Split ring commutator 4) Brushes contacts Beginning of cycle : - Coil is parallel to field Direction of current A→B→C→D (using FLHR) AB experiences a downward force CD experiences an upward force No force is acting on BC & AB because they’re parallel to field → Coil rotates in anticlockwise direction Magnet is curved so field is always ⦜ to coil while rotating) After ¼ Cycle: - Brushes are in line with the gaps - No current in coil - Coil will keep rotting at consent speed, by N1 law. After ½ Cycle: - Coil is parallel to field again - Split rings change contact on brushes: L in contact with Y & M in contact with X - Direction of force is reversed Direction of current D→C→B→A AB experiences an upward force CD experiences a downward force No force is acting on BC & AB because they’re parallel to field → Coil keeps rotating in anticlockwise direction Efficiency of motor = Output KE/Input electrical energy x100 (Or) = Mechanical output power/Input electrical power energy x100 What is the function of the commutator? → Reverses the direction of the current every half cycle, so the forces change direction and keep the coil turning in one direction. The gap between the rings separates the lines of action, so forces don’t cancel out What is the function of the brushes? → The brushes rub against the commutator to transfers the electrical energy from battery to the coil. Why are the brushed usually made of carbon? → Because carbon is a good electrical conductor and a good and lubricant to minimise friction with the commutator The structure of the motor is very similar to that of an a.c. generator. Use ideas about induction to suggest why the current from the battery falls as the motor speeds up → When the motor speeds up, back/opposing e.m.f. increases. At the instant when the coil is vertical, the springy contacts do not, in fact, make contact with the ends of the coil. Describe and explain what happens to the coil. → By N1 Law the coil will keep rotating at constant velocity since no net force acting on it so it will continue turning (inertia) Explain why the coil rotates when the switch is closed (Current passes) → Field around wires AB and CD, Interacts with the field of the magnet. Producing forces on AB and CD that are opposite (up and down) and not in same line of action. So, cause moment in same direction. So, coil rotates rotation Explain why the forces on AB and CD cause the loop to rotate about the axis. → Forces on AB and CD are opposite (up and down) and not in same line of action So, cause moment in same direction. So, coil rotates The Magnetic effect on a moving charge Particle has to be moving and charged. If in time (t), the charge will move a distance (L) L= v x t & I=q/t & F=BIL F= B.q/t.v.t = Bqv F=Bqv (or) F=Bqvsinⲑ → If the particle move at right angles to filed, using FLHR the force will always be at right nags to the direction of motion. So, Particle will move in a circle. mv2/r = Bqv p = Bqr r= p/bq= mv/Bq T= 2𝛑r/v → T=2𝛑m/bq Radius 𝜶: - Momentum - 1/ Flux density - 1/Charge → Explaining the shape of path: - Circular motion in the vertical plane/direction - Using FLHR, there is a vertical force on the electron into page - Force perpendicular to the path acts as centripetal - Component of motion/velocity in a horizontal plane is parallel to the field - No force is horizontal direction Or uniform motion in horizontal direction Or constant velocity in horizontal direction Describe how the path would be different if the electron entered the magnetic field at an smaller angle: - The circles would have a smaller radius - Distance between adjacent loops would increase Devices Velocity selector → Selects or filters particles of a certain velocity, using perpendicular electric and magnetic fields to produce opposing forces to control particle in a vacuumed chamber. Forces only balance a certain velocity, so these particles are undeflected. FB=FE → Equal and opposite in direction Bqv= Eq Bv = E v = E/B (Ratio of the electric and magnetic field strength) → If a particle has a speed greater or less than v, the magnetic force (only magnetic field) will deflect it and collide with one of the charged plates → The electric field does not depend on the velocity F=Eq, however the magnetic field does F=Bqv. (The gravitational force on the charged particles will be negligible compared to the electric and magnetic forces and therefore can be ignored in calculations) Deflection tube → A device used to find the velocity of an electron. By letting electron beam pass through parallel charged plates, cathode (+v) and anode (-ve). Perpendicular magnetic field is applied to them. When the electron beam remains straight, the electric and magnetic forces on them are equal in magnitude but opposite in direction FB=FE Bqv= Eq v = E/B v = V/Bd V→ Voltage between plates d→ Distance between plates The magnetic field is provided by two coils (Helmhol coils) which provide a uniform field between them Mass Spectrometer → A device used to find the mass-to-charge ratio of chemicals (Chemical is ionised before setting machine). The particles will move in a circular path when passing through the magnetic field. Only particles with a certain q/m will reach the detector. FC=FB mv2/r = BqV q/m = v/Br Each chemical has its own unique mass-to-charge value, So this device is used to identity unknown chemicals. (Used in forensics) The mass-to-charge ratio of an electron: e/m = v/Br, 1.76x1011 Ckg-1 → B and r are known from the calibration of the machine, W need to know is how fast the particles were moving when they entered the electromagnet. They are accelerated to this speed by an electric field acting on their charge, and the kinetic energy gained comes from the potential difference,V, that they pass through. ½mv2 = qV v= √2qV/m →Thus, we adjust accelerating and strength of electromagnet q/m = 2V/B2r2 Deriving the magnetic force on conductor using magnetic force on particle: → Consider a metal conductor of length L with electrons travelling along it with an average drift velocity v. If the electrons transverses for time of t, it moved a distance of L. t = L/v & I=q/t & q=ne I= ne/ L/v = nev/L & F=Bqvsinⲑ F=Bnevsinⲑ → BIsinⲑ Electromagnetic induction 1. Faraday’s Law → The induced emf in a circuit is equal to the rate of change of magnetic flux linkage through the circuit or is equal to the rate of cutting of magnetic flux 2. Lenz’s Law → The direction of the induced emf is such that it tends to oppose the flux change causing it, and does oppose it if induced current flows → Combining both laws: Emf (𝛌) = -N.Δɸ/Δt - 1 volt = 1 Wb.s-1 - Emf is only induced when there is change in flux or cutting of flux - Induced current only flows when the circuit is closed - The greater rate of cutting, the greater the emf induced Increasing induced emf: - Moving wire faster - Stronger magnet → Because of the greater rate of change in magnetic field → Fleming right hand rule: (Movement → emf induced, KE caused induction) First finger: Direction of field Second finger: Direction of induced current Thumb: Direction of force (Thrust) → Due to Lenz’s law Primary coil: Solenoid connected to supply Secondary coil: solenoid connected to device When the switched is closed, galvanometer deflects in a given direction and back to zero: → Because the magnetic flux lines are cut, so switch is closed Switch is left closed, there is no deflection: → Because, no find lines are cut. So no emf induced. When the switched is opened, galvanometer deflects in a opposite direction and back to zero: → Field lines are cut by coil in opposite direction, so current is induced in the opposite direction When the switched is closed now, galvanometer shows greater deflection: → Most of the flux pass through soft iron core and reach the other coil. Less flux loss. So, greater rate of cutting of magnetic flux. Greater emf induced For continuous current to be induced: - Use variable resistor to vary current - Switch off and on repeatedly - Replace DC source by an AC source → Because there will be a continuous change in magnetic flux Main points: 1. Change in flux 2. Emf is induced 3. State Faraday’s law 4. If circuit is closed, current flows 5. State Lenz’s law Applications of Lenz’s law Dropping a magnet down a copper tube - If you drop a magnet down a copper tube, it falls more slowly than if you drop a similarly sized non-magnetic piece of metal through it. As copper is non-magnetic piece of material, the friction forces should be identical on the two falling objects. - As the magnet falls, it will induce an emf in copper tube, which causes current to flow around the tube, because the tube is a closed circuit. - The induced current will generate an electromagnetic field, which will interact with the falling magnet. - The direction of an induced emf is such as to oppose the change creating it. - Hence, the emf in the tube acts to try and slow down the magnet If the magnet speeds up magnet would end up with more KE than the GPE it had at the start, which is WRONG according the law of conservation of energy →There is always emf due to the battery Pushing bar magnet through a coil - When a bar magnet is pushed into a coil connected to an ammeter the meter deflects. - When its pulled out of the coil the meter deflects in the opposite direction. - The induced current passing round the circuit creates a B field around the coil. → The coil field must act against the incoming north pole otherwise it would pull the North Pole in faster. - As magnet enters coils a current is induced producing a B field in the coil. The top of the coil becomes a north pole, opposing the motion of the bar magnet. - As the magnet leaves its south pole induces a north pole on the bottom of the coil Bar magnet falling through a coil - As the magnet enters it generates a current in the loop that sets up a magnetic field to oppose the entry of the magnet. - When the magnet is in the middle of the coil it is at the point where the magnetic poles will switch. At this point there is no current flowing in the coil since the p.d is zero. - When exiting the coil the magnet is moving faster (because of its acceleration due to gravity) and it induces a current in the loop that sets up a magnetic field to oppose the magnet moving away i.e. the magnetic poles of the coil change. -Induced emf 𝜶 The rate of cutting of flux linkage -When the magnet gets closer to the coil, there is an initial increase in emf. Due to the change in magnetic flux linkage. -While The magnet is going through the coil, the induced emf decreases. -The speed of magnet increases as it falls and the induced emf reverses reaching a -ve max. value which is higher than the +ve max. value -The time for -ve pulse is shorter Than The time for +ve pulse, because magnet is speeding up constant. Since, Emf (𝛌) = -N.Δɸ/Δt - The areas of the two parts of the graph will be The Same because N.ɸ is → The induced emf as the magnet enters the coil is lower than when it exits the coil. This is because the magnet is moving faster as it exits and thus the rate of change in magnetic flux linkage is greater when it exits. The induced emf changes direction because the current and the magnetic fields both switch to oppose the changes occurring. If the induced current was in the opposite direction, it would attract the magnet into the coil and generate electricity with no energy input, which is AGAINST the law of conservation of energy → Lenz’s law is a re-statement of the principle of conservation of energy; The induced current opposes the motion of the magnet so work has to be done to move the magnet against the induced magnetic field. This work is the energy transfer to the circuit needed to cause a current. Reversing direction of the induced current: - Reversing the magnet - Direction of movement of the magnet Bar magnet falling through a coil, suspended form spring - There is a changing magnetic field/flux linked with the coil when the coil cuts flux - e.m.f. induced - The induced emf depends on rate of change/cutting of magnetic flux - When the magnet is stationary (when the magnet is at maximum displacement) the induced e.m.f. = 0 - The induced emf is positive or negative depends on direction of oscillation of the magnet up and down. - Frequency / T of oscillation of the spring/magnet matches frequency /T of the induced e.m.f. variation - Frequency of induced emf = the frequency of oscillating magnet/spring Spark coil → When the switch is opened, a large potential difference is produced across S and a spark is observed across G - Current in primary coil (P) produces a magnetic field - When switch opened the current in primary coil falls and there is a change in magnetic flux linkage in the secondary coils (S) lines of flux cut the secondary coils - E.m.f. is induced in the secondary coil - Therefore, P.d is produced across the ends of the secondary coil G, hence spark is produced Bicycle Dynamo - The core gains a magnetic field - when the bar magnet rotates the magnetic field lines are cut by the coil of wire - so there is a changing magnetic field - voltage induced across coil - causing a current in the coil of wire → Disadvantages: - The dynamo-wheel friction makes bicycle harder to pedal - The lights would vary in brightness depending on the speed of the bicycle. - lights will be off when bicycle is stationary Motor =E+𝒱 Net voltage= Supply emf + emf induced emf 𝜶 rate of change in magnetic flux linkage - Emf induced is dependant on speed of motor - - If speed starts to decrease, emf induced decreases - Net voltage across the coil of motor will increases - Therefore current in the coil of motor increases I= E-𝒱/R, when coil at rest 𝒱=0 so, I=E/R. I is max - And the speed of motor remains constant → Protective resistor should be installed in series with coil - Protective resistor can be removed when motor is running. This is way a DC motor that is running should never be stopped with the supply, connected. - If this is done emf induced will decrease to zero, I will be very large. Coil may burn off. Applications of Faraday’s law → If a length of wire (L), is a part of a complete citrus cutting through a magnetic field of flux density (B) at right angles. The conductor exercises a force, F=BIL, which opposes the motion. Work-done = Fs = BILΔs 𝛌= W/Q = BILΔs/it W=BILΔs & Q=IT & W=QV 𝛌= BLv AC generator Kinetic/Mechanical energy → Electrical energy Structure: 1) Magnet 2) Coil of wire around a soft core iron 3) Slip ring 4) Brushes contacts → How does it work? - Coil is rotated in magnetic field - There is change in magnetic flux linkage - Emf is induced - According to Faraday’s law, the emf induced is equal to the rate of change in magnetic flus linkage - Since the circuit is closed, current is induced - Direction of induced current can be determined using FRHR Instantaneous emf induced: - Emfmax = NABw (or) =NABWsinⲑ ⲑ → Angle between coil and field 𝛌 = NABwsinwt = NABwsin2𝛑f ⲑ=wt & w=2𝛑f Max Flux: Smallest Flux: When coil is perpendicular to field When coil is parallel to field Gradient = 0 Gradient = max Zero emf: Zero emf: Because it is the gradient of Nɸ - t Because it is the gradient of Nɸ - t graph graph → No magnetic field lines are cut → Greatest rate of change in magnetic (max) value obtained every half cycle linkage From perpendicular to parallel position, its ¼ revolution/cycle Changes in graph if the coil spins faster: -Smaller period -Higher frequency -higher maximum current or greater amplitude Increasing emf induced: 1. Increasing number of coils 2. Increasing coil area 3. Greater magnetic flux density - Stronger magent - North and south pole made closer 4. Coil rotates faster, higher frequency (𝜺𝜶f) - Soft iron core Methods 1,2, and 3 can increases emf induced without affecting the frequency Transformers → How does it work? - Alternating current is supplied to primary coil - As The Current is constantly varying so magnetic flux linkage is produced that is constantly varying - The changing magnetic flux linkage passes through The soft iron core to cut secondary coil - Emf induced in the secondary Coil - The emf induced has some frequency of AC suppled to primary coil → Vp/Vs = Np/Ns The frequency of Input voltage and output voltage is always the same A. Step up transformers: To step up voltage and step down current Ns > Np So, Vs > Vp B. Step down transformers: To step down voltage and up current Ns < Np So, Vs < Vp Power and efficiency - If transformer is 100% efficient:- Pin = Pout V 𝜶 1/I, P is constant Vp.Ip = Vs.Is → Vp/Vs = Is/Ip - If transformer is not 100% efficient:- Efficiency= Pout / Pin x100 = Vs.Is / Vp.Ip x100 Uses of Step up transformers: → Used at power stations, so voltage is stepped up and current is stepped down. So, less power lost as heat. Since, P=I2R Advantages of high voltage power transmission: - Current is very small → Power lost as heat is reduced - Thinner cables can be used → Less costly, less materials used - Less massive pylons → Less costly Advantages and disadvantages of using thicker cables in power transmission: - A: Resistance is reduced, so less power lost as heat. Since, P=I2R - D: Heavier cables and more pylons, more materials, more costly Power lost in transformers (1) Power lost as heat → Due to resistance in the primary and secondary coils Reduce: - Thicker copper wires There is heat loss from long cables, because R 𝜶 Length of wire. So the longer, the ↗ R Thicker for less R, copper for having lower R than other materials (2) Power lost due to flux linkage Reduce: - Wind the secondary coil around the core of primary coil (3) Power lost heat due to eddy currents → Current circulating in the soft iron core Reduce: - Soft iron core must be laminated, so I↘︎, power lost has heat is reduced Why can’t powerhouses become closer: - Magnetic pollution is dangerous Why don’t transformers work with DC supply: - No change in magnetic flux - No electromagnetic induction Why is a soft iron core used: - Magnetises and de-magnetises easily - A hard iron core, will require energy to magnetise, deducting from the output. Reducing the efficiency of transformer Using Lenz’s law to explain input P.d and emf are not in phase: - The induced current in the secondary coil produces a magnetic field that opposes the changing magnetic field produced in the primary coil. - So, the input potential difference and output e.m.f are not in phase. Variation of current, magnetic field in primary coil and emf in secondary coil → Phase difference between I in primary and emf in secondary is 90° Lenz's law also applies in transformers in that the induced current/emf in the secondary coil is always opposite in direction as the one in primary coil. These cause the 2 current to be 90° out of phase and same frequency

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